Polyamides, useful as molding resins and as film and fiber formers, are made commercially by reacting a diacid or diacid precursor with a diamine. In one process a diacid is initially reacted with a diamine in an aqueous media to form a solid polyammonium salt which precipitates from the reaction mixture. The solid is then heated successively in an autoclave reactor under pressure to control explosive initial dehydration and subsequently under reduced pressure to effect conversion of the polyammonium salt to a polyamide. This requires pressurized equipment and careful process control in order to optimize heat and mass transfer. The high melting solid salt which initially cannot be stirred and undergo almost explosive dehydration as the reactor temperature is increased. The solid salt is converted to a mobile liquid which increases in viscosity as additional water is removed as high pressure steam.
Employing these salt processes for the production of polyamides from low molecular weight, highly volatile diamines has encountered some difficulties in the salt preparation from the diamines.
An alternate approach to the use of autoclaves consists of dispersing the polyammonium salts in highly polar solvents and effecting dehydration and solubilization by elevating the temperature of the solution. This process eliminates the need for autoclaves but invokes the use of very special high boiling solvents. The product must ultimately be recovered from solution as a solid. The expensive, high boiling, polar and frequently toxic solvents must be completely removed from the product and be very efficiently recycled to control manufacturing costs.
In still another version, the diacids may be first converted to diacid chloride derivatives which can be reacted in solution or interfacially with diamines at relatively low temperatures to yield polyamides. Very corrosive hydrogen chloride is produced as a by-product of this reaction. The by-product acid must be completely removed from the solution or dispersion of polymer and the polymer must be separated from the reaction solvent.
Additionally, although melt preparation of polyamides from reactions between diesters of dicarboxylic acids and diamines, for example Fox, et. al., U.S. Pat. No. 4,567,249, is known, low pressure melt preparation of polyamides from highly volatile, low molecular weight, diamines such as 1,3-diaminopropane is not known in the art. Accordingly, one object of the present invention is to provide a low pressure melt process for the preparation of polyamides from (a) a diester of dicarboxylic acid and diamine monomers comprising 1,3-diaminopropane.